​Herbs are usually great for improving health.  But there are times when herbs can be harmful.  Some can interact with certain medications and drugs.  Others are not safe to take when pregnant, breastfeeding, or in the process of conceiving.

Plants contain phytochemicals (plant chemicals) and phytosterols (plant hormones), which can affect the uterus and developing baby.  They can also affect a nursing newborn.  They may cause the uterus to contract.  Some constituents of the herbs can pass into the breast milk as well.

The following is a list of herbs deemed not safe to use during pregnancy.  This list is not comprehensive.  There are other herbs that are not safe, but the ones listed here are the more commonly used ones.

Aloe.  Internal use of aloe can initiate uterus contractions and cause a drop in blood sugar.

Goldenseal.  Can cause uterus contractions.

Licorice. Contains a compound called glycyrrhizin, which depletes potassium.  Low potassium levels can increase the risk of developing preeclampsia (high blood pressure during pregnancy). 

Sage. Contains a phytochemical called thujone, which can initiate a menstrual period.  This might cause a miscarriage. Consuming sage while breastfeeding is not recommended because it can decrease lactation in women. 

There are many herbs for which there is no safety data because research cannot be conducted while a woman is pregnant.  It is imperative that you consult a healthcare professional before taking herbs while conceiving, pregnant, or breastfeeding.  Dr. John Phelts, who has been trained in botanical medicine and women's health, provides consultations for pregnant women in his New York, NY office.   

Sources:
Johnson, R.L., S. Foster, Low Dog, T. and Kiefer, D. National Geographic Guide to Medicinal Herbs: The World's Most Effective Healing Plants.(2012) Washington, D.C.: National Geographic

Hess, Henry M. & Miller, Richard, K. Drugs During Pregnancy and Lactation (Third Ed.) Treatment Options and Risk Assessment (2015), "Herbs and Pregnancy" Pages 511-525 https://www.sciencedirect.com/science/article/pii/B9780124080782000202